Preheat your oven to 350°F (175°C).
In a large bowl, combine the cooked rice and chopped broccoli.
Add the shredded cheddar cheese, sour cream, milk, chopped onions, garlic powder, salt, and pepper.
Mix all ingredients until well combined.
Transfer the mixture to a greased baking dish.
If desired, sprinkle breadcrumbs on top for a crunchy texture.
Bake in the preheated oven for 25-30 minutes or until golden brown.
Remove from the oven and let it cool for a few minutes.
Cut into squares and serve warm.
Enjoy your delicious broccoli rice casserole!